1. Field of the Invention
The present invention relates to prosthetic joint devices and, more particularly, to a joint prosthesis having an infinitely variable head.
2. Background Information
The state of the prosthetic joint market has progressed such that a surgeon generally approaches joint replacement surgery in just one of two strategic manners. This is especially true with respect to the prosthetic shoulder market. With respect to a humeral replacement procedure, in one strategic manner a surgeon is provided with instrumentation and technique guidelines for the particular shoulder prosthesis or prosthesis line. The guidelines and/or instrumentation direct or dictate the angle of humeral head resection for the implant (prosthesis). This angle is in relation to the humeral intramedullary (IM) canal and is designed to match an optimum set of angles already present in the prosthetic design.
Another strategic manner is to perform the shoulder replacement surgery in accordance with a patient's anatomy. Particularly, the humeral head is resected according to angles perceived to be “anatomic” in the opinion of the surgeon, not according to angles already present in the prosthetic design. The prosthesis is designed such that the configuration of the prosthesis is intra-operatively adjustable (i.e. adjustable during the surgical implantation thereof). This type of configuration allows the prosthesis to be adjustable such that is can match the surgical boney preparation.
Even with respect to these two divergent manners of surgical strategy, a common problem in shoulder surgery is matching the humeral resection angle to the predetermined angle designed into the prosthesis. This angle may be described as the angle between a prosthetic collar and a diaphyseal section of the stem. In the case of a collarless stem, the angle may describe the difference between a long axis of the stem and an inferior surface of the prosthetic head. It is considered optimal for fixation and biomechanics if the resected angle and the angle of the prosthesis are identical—thereby allowing intimate contact between the superior surface of the resected bone and the inferior surface of the implant.
Moreover, the angular version in which the prosthesis is implanted will have a significant impact on the biomechanics of the prosthetic joint. Currently, most shoulder prosthesis systems on the market dictate the varus/valgus angle of the bone cut. This strategy does not allow the surgeon to easily alter biomechanics after the prosthesis has been trialed, much less implanted.
There are two known products currently marketed that attempt to resolve at least one of the above-noted issues.
A first product currently marketed as a solution to the problems addressed above is the CenterPulse Anatomica based on U.S. Pat. No. 5,741,335 issued to Gerber. This product provides a humeral head that is infinitely adjustable in varus/valgus and anterior/posterior angles relative to the stem portion of the prosthesis. This is accomplished through a spherical shaped protrusion on the superior surface of the stem that fits into a spherical recess in the humeral head. These mating surfaces allows the head to be articulated about the stem, thus allowing adjustable positioning of the head. The head can be locked in a position relative to the stem. This solution provides adjustment of the neck-shaft angle as well as being able to affect adjustment of the version through flexibility in the anterior/posterior angle. The locking means, however, is sub-optimal. Particularly, the locking mechanism, requires the turning of a locking screw that has its head facing lateral and inferior, for which there is no access once the stem has been cemented. This eliminates the ability to adjust head position once the stem has been implanted, and forces a total revision if articular surfaces ever need to be revised. Lastly, the protrusion on the humeral stem even when the humeral head is not in place limits the surgeon's access to the glenoid in preparation for a glenoid replacement.
A second product, the Tornier-Aequalis system (based on U.S. Pat. No. 5,702,457 issued to Tornier) provides a modular junction within the metaphyseal region of the stem which allows a small block between the stem and humeral head to be interchanged. This block is available in multiple angles, thus allowing the surgeon to select the block that best fits the boney anatomy as resected. This system, however, has two primary weaknesses. First, the use of modular blocks obviously forces the design to only allow angular adjustments in finite increments. Second, the need to adjust the angle through modular blocks forces the surgeon to remove the stem, change out a component, and reset the stem. This presents inconvenience, as well as risk for interfering with resected bone and compromising fixation.
A problem with the above-identified and other prosthetic systems, is the need for complicated instrumentation for correctly orienting and/or adjusting the final orientation of the head portion of the prosthesis. Moreover, since fixation for the above-identified and other prosthetic systems is provided by mechanisms that are outside of the humeral stem, these systems provide glenoid exposure problems.
What is thus needed in view of the above is a joint prosthesis that allows adjustment of the angular position of the head of the joint without utilizing a separate fastener or locking device to fix the position of the head.
What is thus further needed is a joint prosthesis that allows easy adjustment and locking of the angle of the head during surgery.
What is thus even further needed is a joint prosthesis that allows adjustability of the joint head after final implanting of a mating joint prosthesis stem.